Expt: Determining Relative Density of a Liquid using Hydrometer

Aim: To determine the relative density of a liquid with respect to the density of water using a hydrometer.

Hydrometer: A hydrometer is an instrument used to measure relative density of two liquids. It works on the principle of flotation, such that the weight of the liquid displaced by the hydrometer is equal to its weight. The height at which the hydrometer floats in a liquid is inversely proportional to the density of that liquid.



  1. Take a flat test-tube and stick a paper scale on the inside wall with the zero marking at the bottom.
  2. Add some lead balls, so that the test-tube gains weight.
  3. Take some water in a jar and immerse the hydrometer in water.
  4. Note the height at which the hydrometer floats in water. Record this as the reference reading.
  5. Immerse the hydrometer in some other test liquid and note the height at which it floats.
  6. Calculate the relative density of the liquid using the equation provided.


Relative Density of Liquid = Density of Liquid/Density of Water

(also) Relative Density of Liquid = Float Height of Hydrometer in Water/Float Height of Hydrometer in Liquid